Note: This is pure and simple personal opinion. Please understand that I do not judge anybody's character based on whether they are vegetarian or non-vegetarian. It does not affect my friendships or interactions, in the slightest way. Nor do I try to influence anybody towards vegetarianism. I am simply reasoning why I lean towards vegetarianism. That is all.
Why am I a vegetarian?
The obvious answer would be, "Because I was born into a pious Brahmin family." If that were the reason, I would go nuts at the very sight of non-vegetarian food. Yet, I never mind when my friends eat non-vegetarian. I am passionately vegetarian. Period. (I do confess to eating pastries occasionally, although I am haunted in my dreams by the vision of a pretty yellow chick which cries, "I am the chick that never was.")
My points in favour of vegetarianism extend beyond birth and social conventions. I am completely aware of the fact that Hinduism is not a religion but a way of life. It lays down certain tenets for an ideal life. Two of those which I vehemently subscribe to are vegetarianism and teetotalism. Of course, I am also against smoking, but fortunately or unfortunately I think there were no cigars at the time when the Manusmriti was formulated, so it is not explicitly mentioned.
You may be interested in noting that Hinduism does not decree non-vegetarianism or alcoholism as sins. It just mentions that they are low in the scale of goodness. Abstinence has its benefits - in terms of better physical and mental health (and today it's been proved scientifically too.)
Health apart, the very thought of killing an animal for my food is nauseating. If you think poultry farms wait till the chicken die naturally before shipping them, I am sorry but you are wrong. Gruesome to even think about, but I have (unintentionally) seen how they break the neck bone and dip the innocent bird in hot turmeric water before plucking its feathers off. God! I have seen this scene over and over again in Yercaud, one of the beautiful hill stations in Tamilnadu, during my morning walks. In Chennai, there used to be a hotel in the street immediately behind our home, and I have heard the birds scream at the face of death.
Close your eyes for a moment, forget that they are birds, and you will actually hear the screams of a tormented man. Maybe because they could sense the impending doom, the birds used to start screaming even as the kitchen assistant's hands reached into the cage! If you have seen this, even once in your life, you will never feel the desire to eat non-vegetarian food.
In my food habits, I am not even close to the satvik levels of Jain food (they don't even consume root vegetables because several bacteria are killed when the tuber is pulled out; moreover, in the case of tubers, the plant also dies when it is pulled out) or vegan food (they do not consume even dairy products because you are actually taking away what belongs to the calf). Today, I am just a simple vegetarian, but I would like to achieve these levels one day.
Even at my plane of vegetarianism, I can at least sleep well, because when vegetables and fruits are plucked, plants don't scream out in pain. They can grow more fruits and vegetables. And the plants continue to live, except in the case of tuber vegetables and grains (and even there, I am afforded the satisfaction of knowing that even if only for a living, the farmers plant a new sapling as soon one is plucked out).
A funny thought often crosses my mind: when a man-eater roams into a village and hits a man, the whole village starts hunting for it in a frenzy and rests only after it is killed. If the animals got together like this and started attacking every man who chops an animal? ;-)
Before I sign off, here's some food for thought. The rhinoceros, one of the strongest mammals, is a vegetarian!